1. Technical Field
The present invention relates to capacitance type vibration sensors, and in particular, to a vibration sensor of a microscopic size manufactured using an MEMS (Micro Electro Mechanical System) technique or a micromachining technique.
2. Background Art
Basic Structure of Vibration Sensor
FIG. 1 shows a basic structure of a capacitance type vibration sensor. A vibration sensor 11 has a vibration electrode plate 14 arranged on an upper surface of a substrate 13 having a hollow portion 12 at a central part, and an upper side of the vibration electrode plate 14 covered with a fixed electrode plate 15. A plurality of acoustic perforations 16 (acoustic holes) pass through the fixed electrode plate 15 in an up and down direction. A vent hole 17 is provided between the upper surface of the substrate 13 and the lower surface of the vibration electrode plate 14 at the periphery of the hollow portion 12, where a space (hereinafter referred to as air gap 18) between the vibration electrode plate 14 and the fixed electrode plate 15 and the hollow portion 12 are communicated by the vent hole 17. The vibration sensor (capacitor microphone) of such structure is disclosed in patent document 1.    Patent Document 1: Japanese Unexamined Patent Publication No. 2004-506394
When an acoustic vibration 19 is propagated through air towards the vibration sensor 11, the acoustic vibration 19 passes the acoustic perforation 16 and spreads through the air gap 18 thereby vibrating the vibration electrode plate 14. When the vibration electrode plate 14 vibrates, an inter-electrode distance between the vibration electrode plate 14 and the fixed electrode plate 15 changes, and hence the acoustic vibration 19 (air vibration) can be converted to an electric signal for output by detecting the change in electrostatic capacitance between the vibration electrode plate 14 and the fixed electrode plate 15.
Action of Vent Hole
In such vibration sensor 11, the hollow portion 12 is formed in the substrate 13 so that the surface of the substrate 13 does not interfere with the vibration of the vibration electrode plate 14. The hollow portion 12 may pass through the substrate 13 in the up and down direction as in FIG. 1, or may be a recess blocked by the lower surface of the substrate 13. In the case of the hollow portion 12 that passes therethrough, the lower surface of the through-hole is often blocked by a wiring substrate and the like when the vibration sensor 11 is mounted on the wiring substrate, and the like. Thus, the hollow portion 12 is sometimes called a back chamber.
Since the lower surface of the hollow portion 12 is often substantially blocked, a pressure in the hollow portion 12 sometimes differs from an atmospheric pressure. Furthermore, the interior of the air gap 18 may also differ from the atmospheric pressure due to the ventilation resistance of the acoustic perforation 16. As a result, a pressure difference occurs between an upper surface side (air gap) and a lower surface side (hollow portion 12) of the vibration electrode plate 14 from a peripheral air pressure fluctuation, temperature change, and the like, thereby bending the vibration electrode plate 14, and possibly creating a measurement error in the vibrations sensor 11.
Therefore, in the general vibration sensor 11, the vent hole 17 is provided between the vibration electrode plate 14 and the substrate 13 as shown in FIG. 1 to communicate the upper surface side to the lower surface side of the vibration electrode plate 14. As a result, the pressure difference between the air gap 18 and the hollow portion 12 is removed, and the measurement accuracy of the vibration sensor 11 is enhanced.
Furthermore, as the area of a fixing site of the vibration electrode plate 14 to the substrate 13 can be reduced by providing the vent hole 17, the vibration electrode plate 14 becomes flexible and the sensor sensitivity can be enhanced.
Regarding Noise by Thermal Noise
In the vibration sensor described above, the output signal contains noise, which lowers an S/N ratio of the sensor output. The inventors of the present invention searched for the cause of the noise of the vibration sensor, and found that the noise generated in the vibration sensor originates from the thermal noise (fluctuation of air molecules) in the air gap between the vibration electrode plate and the fixed electrode plate. In other words, as shown in FIG. 2(a), the air molecule α in the air gap 18 between the vibration electrode plate 14 and the fixed electrode plate 15, that is, the quasi-sealed space, impinges on the vibration electrode plate 14 due to fluctuation. The microscopic force caused by the impact with the air molecule α is applied on the vibration electrode plate 14, and the microscopic force applied on the vibration electrode plate 14 randomly fluctuates. Thus, the vibration electrode plate 14 microscopically vibrates by thermal noise, and the electric noise is generated in the vibration sensor. In particular, the noise caused by such thermal noise is large and thus the S/N ratio degrades in the vibration sensor (microphone) of high sensitivity.
The inventors of the present invention thus proposed escaping the thermal noise (air molecules) generated in the air gap between the vibration electrode plate and the fixed electrode plate from the acoustic perforation to reduce the noise caused by the thermal noise (Japanese Patent Application No. 2008-039048).
However, it was found in the subsequent research that the noise caused by thermal noise is generated not only in the air gap 18 but also in the vent hole 17, and that the noise caused by the thermal noise in the vent hole 17 occupies a considerable proportion of the noise component. In particular, the noise by the thermal noise is likely to occur since the vent hole 17 has a smaller gap compared to the air gap 18.
Therefore, the noise by the thermal noise in the vent hole needs to be reduced in the vibration sensor including the vent hole. The method of reducing the noise by the thermal noise includes methods of widening the gap of the vent hole, shortening the length of the vent hole in the ventilation direction, and enabling the air molecules, which become the cause of thermal noise, to easily escape from the vent hole 17.
Relationship of low frequency characteristics and acoustic resistance
The low frequency characteristics of the vibration sensor will now be described. The vent hole formed between the substrate and the vibration electrode plate communicates the upper surface side to the lower surface side of the vibration electrode plate, as described above, to reduce the pressure difference. However, if the gap of the vent hole is large, the acoustic resistance of a path (shown with an arrow line 20 in FIG. 1) from the acoustic perforation in the vicinity to the hollow portion of the substrate through the vent hole becomes small. Furthermore, a low frequency vibration of the vibration passed through the acoustic perforation and propagated into the air gap easily leaks out to a hollow portion side through the vent hole since the low frequency vibration easily passes through the vent hole compared to the high frequency vibration. As a result, the low frequency acoustic vibration that passes through the acoustic perforation near the vent hole leaks out to the hollow portion side without vibrating the vibration electrode plate, which degrades the low frequency characteristics of the vibration sensor.
In the frequency characteristics of the sensor sensitivity, the limiting frequency at which the sensor sensitivity starts to lower when the frequency becomes lower than such a frequency is called the roll off frequency fL. The roll off frequency fL of the vibration sensor is expressed with the following equation 1.1/fL=2π·Rv(Cbc+Csp)  (equation 1)
Where                Rv: acoustic resistance (resistance component of vent hole)        Cbc: acoustic compliance of the hollow portion of the substrate        Csp: stiffness constant of the vibration electrode plate        
Therefore, the roll off frequency fL is desirably made as small as possible to reduce the lowering of the sensor sensitivity in the low frequency region. According to one or more embodiments of the present invention, about fL=50 Hz is preferable.
It can be recognized from equation 1 that the value of the acoustic resistance Rv of the vent hole is to be made large in order to reduce the roll off frequency fL and reduce the lowering of the low frequency characteristics of the vibration sensor.
The acoustic resistance Rv of the vent hole is expressed with the following equation 2.Rv=(8·μ·t·A2)/(Sv2)  (equation 2)
Where                μ: viscosity coefficient of air        t: length of the vent hole in ventilation direction        A: area of a diaphragm        Sv: cross-sectional area of the vent hole        
Therefore, the length t of the vent hole in the ventilation direction is to be made long or the cross-sectional area Sv of the vent hole is to be made small in order to have a sufficiently large acoustic resistance Rv and reduce the roll off frequency fL.
Relationship of noise by thermal noise and low frequency characteristics
The following conclusion can be derived by summarizing the above. The gap of the vent hole is widened or the length of the vent hole in the ventilation direction is shortened in order to reduce the noise by the thermal noise in the vent hole. The length t of the vent hole in the ventilation direction is made large or the cross-sectional area Sv of the vent hole is made small in order to prevent the low frequency characteristics of the vibration sensor from degrading.
If the gap of the vent hole is widened or the length of the vent hole in the ventilation direction is shortened in order to reduce the noise by the thermal noise in the vent hole, the low frequency characteristics of the vibration sensor degrade. If, on the other hand, the length t of the vent hole in the ventilation direction is made large or the cross-sectional area Sv of the vent hole is made small in order to prevent the low frequency characteristics, the noise by the thermal noise of the vent hole increases, and the S/N ratio of the vibration sensor degrades.
Due to such reasons, the lower noise of the vibration sensor and the satisfactory low frequency characteristics are in the trade off relationship in the structure of the conventional vibration sensor, and it is difficult to manufacture a vibration sensor that is of low noise and that has satisfactory low frequency characteristics.